1. Field of the Invention
The present invention relates to a multilayer piezoelectric device and a method of producing the same. More particularly, the present invention relates to a multilayer piezoelectric device used for producing a piezoelectric actuator including a plurality of separately drivable actuator units and a method of producing the multilayer piezoelectric device, and a piezoelectric actuator including such a multilayer piezoelectric device.
2. Description of the Related Art
Conventional print heads in inkjet printers generally are driven using a piezoelectric actuator. An example of such a piezoelectric actuator is disclosed in Japanese Unexamined Patent Application Publication No. 11-320881. The piezoelectric actuator described in this publication has an external shape shown in FIG. 13 and includes a multilayer piezoelectric device 31.
The multilayer piezoelectric device 31 includes a piezoelectric layered body 37, which is a rectangular ceramic sintered body, including a driving section 34 and a connecting section 36. In the driving section 34, a plurality of first driving internal electrodes 32 and a plurality of second driving internal electrodes 33 are alternately placed upon each other through a piezoelectric layer. In the connecting section 36, a plurality of connecting internal electrodes 35 are placed upon each other through a piezoelectric layer. The piezoelectric layer of the driving section 34 is a portion that stretches and contracts along a lamination direction, called a d33 direction, in response to application of an alternating voltage thereto after it has been polarized.
As shown in FIG. 13A, which illustrates the multilayer piezoelectric device 31 viewed from one side thereof, end portions of the first driving internal electrodes 32 are exposed from only one of opposing side surfaces 37a and 37b of the piezoelectric layered body 37, that is, from only the first side surface 37a, whereas end portions of the first driving internal electrodes 32 are not exposed from the second side surface 37b. As shown in FIG. 13B, which shows the multilayer piezoelectric device 31 viewed from the other side thereof, end portions of the second driving internal electrodes 33 are exposed from the second side surface 37b, whereas end portions are not exposed from the first side surface 37a. End portions of the connecting internal electrodes 35 are exposed from both the first side surface 37a and the second side surface 37b of the piezoelectric layered body 37.
Driving-side external electrodes 38 and connection-side external electrodes 39 are each formed in parallel at the first side surface 37a of the piezoelectric layered body 37 through a gap 40 of a predetermined width. The first driving internal electrodes 32 and the connecting internal electrodes 35 are separately in electrical conduction with their corresponding external electrodes 38 and 39. A common-side external electrode 41 is disposed at the second side surface 37b of the piezoelectric layered body 37. The second driving internal electrodes 33 and the connecting internal electrodes 35 are all in electrical conduction with the common-side external electrode 41.
Here, the common-side external electrode 41 is formed over the entire second side surface 37b, while the driving-side external electrodes 38 and the connection-side external electrodes 39 are formed after providing a gap 42 of a predetermined width in the bottom portion of the first side surface 37a, that is, after being separated through the gap 42 from the bottom surface of the piezoelectric layered body 37. Therefore, as shown in FIG. 14, the driving section 34 of the multilayer piezoelectric device 31 has a cross-sectional structure such as that shown in FIG. 14.
A piezoelectric actuator 45 having an external form shown in FIG. 15 is produced using the multilayer piezoelectric device 31. More specifically, the multilayer piezoelectric device 31 is secured to a supporting substrate 46 using, for example, an adhesive, and a plurality of slits 47, which extend in a lamination direction X from the top surface to the bottom surface of the driving section 34, are formed in the driving section 34 of the piezoelectric layered body 37 of the multilayer piezoelectric device 31.
As shown in FIG. 15A, which shows the piezoelectric actuator 45 viewed from one side thereof, by dividing the first driving internal electrodes 32 and the second driving internal electrodes 33, and the driving-side external electrodes 38 by the slits 47, the driving section 34 is divided into a plurality of driving section portions in order to provide a plurality of separately drivable actuator units 48. Obviously, by these slits 47, the first driving internal electrodes 32 and the second driving internal electrodes 33 that have been placed upon each other in the driving section 34 are divided.
Here, since the driving-side external electrodes 38 are also divided, a plurality of unit external electrodes 49 corresponding to the separate actuator units 48 are formed. The portions between the connecting section 36 and the driving section 34 of the piezoelectric actuator 45 may be divided by the slits 47. FIG. 15B shows the piezoelectric actuator 45 viewed from the other side thereof.
Although not shown, a flexible printed circuit drawn out from a driving signal source, installed externally of the piezoelectric actuator 45, is connected to the piezoelectric actuator 45 including the actuator units 48. By applying alternating voltage between each unit external electrode 49 and the common-side external electrode 41, or, actually, between each unit external electrode 49 and each connection-side external electrode 39, connected to the common-side external electrode 41 through each connecting internal electrode 35, each actuator unit 48 is driven.
As described above, in the multilayer piezoelectric device 31 used for producing the piezoelectric actuator 45, the driving-side external electrodes 38 and the connection-side external electrodes 39 are disposed at the first side surface 37a of the piezoelectric layered body 37, and the common-side external electrode 41 is disposed at the second side surface 37b of the piezoelectric layered body 37. As shown in FIG. 16, when forming these external electrodes 38, 39, and 41, a deposition mask 50 for completely covering portions of the piezoelectric layered body 37 other than the side surfaces 37a and 37b is provided. After placing the piezoelectric layered body 37 inside the deposition mask 50, an electrode formation process, that is, evaporation or sputtering, is carried out. The arrows shown in FIG. 16 indicate the directions of film deposition.
However, when the piezoelectric layered body 37 is placed inside the deposition mask 50, a gap 51 is formed between them, and the deposition mask 50 undergoes thermal deformation at the time of film deposition, so that a film-deposition precision of the order of only xc2x10.1 mm can be obtained. As shown in FIG. 14, the driving-side external electrodes 38 and the connection-side external electrodes 39, disposed at the first side surface 37a of the multilayer piezoelectric device 31 having a height H of 1.0 mm, need to be separated from the bottom surface of the piezoelectric layered body 37 through the gap 42 having a width W of 0.1 mm. Therefore, problems such as those described below arise.
Since a high film-deposition precision cannot be achieved, the width W of the gap 42 becomes large or small, so that the heights of the locations where the driving-side external electrodes 38 and the connection-side external electrodes 39 are disposed become large or small. When the heights of the portions where the driving-side external electrodes 38 are disposed become large, so that the width W of the gap 42 becomes too large, the external electrodes 38 and the first driving internal electrodes 32 may not be in electrical conduction with each other as shown in FIG. 17, so that faulty electrical continuity results.
On the other hand, when the heights of the portions where the driving-side external electrodes 38 are disposed become too small, as shown in FIG. 18, the driving-side external electrodes 38 are not sufficiently divided by the slits 47 used for producing the piezoelectric actuator 45, so that short circuits occur between the unit external electrodes 49. To overcome such problems which must be solved, it is possible to make it difficult for faulty electrical continuity or a short circuit to occur even if good film-deposition precision in the height direction is not achieved by setting a large separation distance (at a lower external layer portion) between the bottom surface of the piezoelectric layered body 37 of the multilayer piezoelectric device 31 and the bottommost ones of the driving internal electrodes 32 and 33.
However, when such a structure is used, not only does the multilayer piezoelectric device 31 become large as a result of increasing the thickness of the lower external layer portion thereof, but also mechanical processability is greatly reduced because it is necessary to cut the slits 47 deeply. In addition, cracks tend to be produced from the thick lower external layer portion, so that the multilayer piezoelectric device 31 tends to be damaged.
Further, in the multilayer piezoelectric device 31, the driving-side external electrodes 38 and the connection-side external electrodes 39 are formed flush with the side surface 37a of the piezoelectric layered body 37, so that, as shown in FIG. 19, a fillet 52 of an adhesive used for securing the multilayer piezoelectric device 31 to the supporting substrate 46 oozes out to the surface of the driving-side external electrodes 38 and the connection-side external electrodes 39 due to surface tension. When the fillet 52 of the adhesive sticks onto the external electrodes 38 and 39, it becomes difficult for solder to stick when soldering a flexible printed circuit to the unit external electrodes 49 and the connection-side external electrodes 39.
In order to overcome the problems described above, preferred embodiments of the present invention provide a multilayer piezoelectric body, and a method producing the same, which is constructed so that faulty electrical continuity between an external electrode and a driving internal electrode and a short circuit at the external electrode do not occur, and so that a fillet of an adhesive does not ooze out to the surface of the external electrode.
According to a first preferred embodiment of the present invention, a multilayer piezoelectric device includes a piezoelectric layered body including a driving section where first driving internal electrodes and second driving internal electrodes are alternately placed upon each other through a piezoelectric layer and a connecting section where connecting internal electrodes are placed upon each other through a piezoelectric layer, in which at least a driving-side external electrode and a connection-side external electrode, which are in electrical conduction with the first driving internal electrodes and the connecting internal electrodes, respectively, are arranged substantially in parallel at one of opposing side surfaces of the piezoelectric layered body, and in which at least a common-side external electrode, which is in electrical conduction with the second driving internal electrodes and the connecting internal electrodes, is disposed at the other side surface of the piezoelectric layered body. In the multilayer piezoelectric device, a plurality of separately drivable actuator units are defined by a slit that divides the driving internal electrodes of the driving section and that extends along a lamination direction from a top surface to a bottom surface of the driving section. In addition, a cutaway portion is formed in the one side surface of the piezoelectric layered body where the driving-side external electrode and the connection-side external electrode are arranged in parallel so that the cutaway portion is substantially parallel to the driving internal electrodes and the connecting internal electrodes, with the cutaway portion having a depth that allows the cutaway portion to extend from a bottom surface of the piezoelectric layered body to a location above a bottom end of the slit but does not allow the cutaway portion to reach the driving internal electrodes.
In this preferred embodiment, a cutaway portion is disposed in one side surface of the piezoelectric layered body of the multilayer piezoelectric device so as to be substantially parallel to the driving internal electrodes and the connecting internal electrodes, with the cutaway portion having a depth which allows it to extend from the bottom surface of the piezoelectric layered body to a location above the bottom end of the slit, but does not allow it to reach the driving internal electrodes of the driving section. In addition, the driving-side external electrode and the connection-side external electrode are disposed at portions of one side surface of the piezoelectric layered body excluding the cutaway portion. More specifically, in the multilayer piezoelectric device, the connection-side external electrode and the driving-side external electrode, which define a unit external electrode of the piezoelectric actuator, are arranged so as to be reliably separated by a predetermined interval from the bottom surface of the piezoelectric layered body through the cutaway portion, that is, by an interval which matches the depth of the cutaway portion.
Therefore, the heights of the portions where the driving-side external electrode and the connection-side external electrode are disposed are not increased or decreased, so that faulty electrical continuity between the external electrodes and the driving internal electrodes and a short circuit between unit external electrodes do not occur. When a cutaway portion is formed in one side surface of the piezoelectric layered body, a fillet of an adhesive for securing the multilayer piezoelectric device to a supporting substrate accumulates inside the cutaway portion, thereby providing the advantage that the fillet does not ooze out to the surface of the driving-side external electrode and the connection-side external electrode.
According to a second preferred embodiment of the present invention, a method of producing the above-described multilayer piezoelectric device of the first preferred embodiment includes the steps of producing a parent substrate where a plurality of piezoelectric layered bodies to be separated by a dividing process are continuously formed in parallel, forming cut-in grooves, which define a plurality of cutaway portions of the corresponding piezoelectric layered bodies, in corresponding areas, which become the piezoelectric layered bodies formed at the parent substrate, separating the piezoelectric layered bodies by dividing the parent substrate, and after supporting the separated piezoelectric layered bodies in a tilted state with respect to an evaporation source or a sputtering source, forming the driving-side external electrode and the connection-side external electrode by evaporation or sputtering.
According to a third preferred embodiment of the present invention, a method of producing the above-described multilayer piezoelectric device of the first preferred embodiment includes the steps of placing upon each other in a lamination direction a plurality of piezoelectric layered bodies that have been separated by dividing a parent substrate where the piezoelectric layered bodies are continuously formed in parallel, forming the driving-side external electrode and the connection-side external electrode on each of the piezoelectric layered bodies, and forming cut-in grooves, which define cutaway portions of the corresponding piezoelectric layered bodies, in the corresponding piezoelectric layered bodies that have been placed upon each other.
According to a fourth preferred embodiment of the present invention, a method of producing the above-described multilayer piezoelectric device of the first preferred embodiment includes the steps of placing upon each other in the lamination direction a plurality of piezoelectric layered bodies that have been separated by dividing a parent substrate where the piezoelectric layered bodies are continuously formed in parallel, forming cut-in grooves, which become cutaway portions of the corresponding piezoelectric layered bodies, in the corresponding piezoelectric layered bodies that have been placed upon each other, and after supporting the piezoelectric layered bodies in a tilted state with respect to an evaporation source or a sputtering source, forming the driving-side external electrode and the connection-side external electrode by evaporation or sputtering.
When any one of the methods of producing the multilayer piezoelectric device is used, the cutaway portions of the piezoelectric layered bodies of the multilayer piezoelectric device of the first preferred embodiment can be very easily formed. In addition, the driving-side external electrode and the connection-side external electrode to be formed over the entire one side surface of each piezoelectric layered body excluding the cutaway portion can be formed simultaneously on the piezoelectric layered bodies.
According to a fifth preferred embodiment of the present invention, a piezoelectric actuator which is produced using the multilayer piezoelectric device of the first preferred embodiment includes a plurality of separately drivable actuator units that are formed by dividing the first driving internal electrodes and the second driving internal electrodes placed upon each other in the driving section by the slit. The driving section of the piezoelectric device is divided by the slit extending along the lamination direction from the top surface to the bottom surface of the driving section.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.